1. Field of the Invention
This invention relates to a method and apparatus for making phase shift masks used in the manufacture of semiconductor microcircuits and other electronic components to transfer a circuit pattern onto a workpiece, and, in particular, to a self-aligned alternating phase shift mask wherein etching of an anti-reflective coating used on the opaque patterns of the mask is avoided.
2. Description of Related Art
In the manufacture of circuit patterns on electronic components such as the manufacture of integrated circuits on semiconductor substrates photomasks are used to transfer the desired circuit pattern onto the substrate workpiece. Typically, a photomask comprises a patterned metal film such as chromium, nickel or aluminum in a thickness of about 1,000 xc3x85 deposited on a transparent base such as glass or quartz. The photomask is generally manufactured by depositing a thin film of the metal on the surface of the transparent substrate, coating an anti-reflective coating on the metal, coating with a photoresist layer, exposing a pattern on the photoresist coating, developing the resist coating, and removing the metal from the unprotected areas of the film by etching leaving patterned metal film on the substrate. Mask blanks are typically purchased containing the metal film, anti-reflective coating and photoresist layer.
The pattern contained in the photomask is reproduced onto the surface of a workpiece typically by placing the mask over the workpiece and irradiating a radiation-sensitive resist material on the workpiece. The varieties of radiation sources for lithography that have been used or proposed for phase shift masks include visible light and ultraviolet light with optical wavelengths of 365 nm, 248 nm, 193 nm and 157 nm. When illuminated by the radiation, the metal pattern on the photomask serves to selectively block portions of the radiation beam while allowing other portions to be transmitted therethrough. In this manner, very complex geometries having very narrow line widths can be reproduced allowing the economical production of very large scale integrated circuits and other devices.
Optical photolithography has been widely used in the semiconductor industry in connection with the formation of a wide range of structures in integrated circuit (IC) chips. As the device density on IC chips has increased, the size of the structures making up the devices has approached the wave length (around 0.25 micron) of the light used in optical photolithography processes. Future structures approach 90 nm images with a 160 nm pitch (line and space) at 193 nm wavelength. Optical diffraction makes it difficult to form images smaller than the wavelength of light when conventional masks and illumination techniques are used. Future density increases in IC devices may be difficult to achieve absent the development of alternative lithographic technologies.
Phase-shift lithography was developed to enhance the resolution of conventional optical photolithography. Phase-shift lithography is based on opposite phase destructive interference of the waves of incident light. By shifting the phase of one region of the incident light waves 180xc2x0, relative to an adjacent region of incident light waves, a sharply defined dark zone is created in the projected image of the phase shift mask. This zone defines an interface with greater edge contrast than can be achieved in the interface between a clear area and an opaque absorber in a conventional mask. Such a phase shift mask is well-known in the art and is termed an alternating phase shift mask (altPSM). The alternating regions that are etched to 180xc2x0 phase are defined by a write operation on the mask. To facilitate proper alignment of the mask pattern to the chrome-on-glass (COG) mask these phase patterns are slightly oversized.
The anti-reflective coating is used on the surface of the blocking (chrome) layer to reduce the scattering of stray light in the optical lithographic process. Stray light in the optical lithography process reduces image contrast by scattering into areas that should be dark. Unfortunately, during the quartz etching process to form the phase shift pattern, exposed portions of the anti-reflective coating resulting from the oversize write operation may become degraded and lost to the quartz etch. This is shown in FIGS. 2A-2E which show a prior art method for making a self-aligned alternating phase shift mask patterning process using a chrome-on-glass mask which is etched to form the phase shift pattern.
Referring to FIG. 2A, a COG mask is shown generally as 10 and comprises a transparent substrate 12, patterned chrome regions 13 on the surface of the substrate 12 and an anti-reflective coating 14 on the surface of each of the chrome regions 13. A photoresist 15 is shown overlying the coated chrome patterns 13 as well as the upper surface 12a of the substrate.
A write operation using an energy beam 18 is directed onto the resist 15 to form the regions 19 which are to be etched to form the 180xc2x0 shift.
Referring to FIG. 2B, after exposure of the mask 10, portions 19 of the photoresist 15 were exposed corresponding to the write operation. It can be seen that the exposed areas 19 extend over the edge of the chrome pattern 13 and overlying anti-reflective coating 14. This is the conventional process used to form a slightly oversized opening in the fabrication of alternating phase shift masks.
In FIG. 2C, the exposed areas 19 are developed forming openings 20 in the resist layer 15. Again it can be seen that the openings 20 overlap the chrome pattern 13 and overlying anti-reflective coating 14.
In FIG. 2D the mask 10 is etched forming phase shift recesses 21 in the substrate 12. The recesses 21 are formed where there are openings 20 in the resist 15 formed from the imaging process. The recesses are formed typically to provide a 180xc2x0 phase shift. It will be noted that part of the anti-reflective coating 14 is also etched as shown at 22 so that the remaining anti-reflective coating 14 does not cover the entire chrome pattern 13 surface. The remaining resist 15 is then removed leaving the final phase shift mask as shown in FIG. 2E. As can be seen, the anti-reflective coating 14 does not completely cover the chrome pattern design 13 and thus reduces the effectiveness of the mask when used to transfer the pattern onto a workpiece. In addition to causing more reflection in the lithography tools, the missing anti-reflective coating that is formed around every phase opening appears as a bright halo in the reflective image capture during mask inspection. The concept of the enlarged phase opening is to provide overlay insensitivity and this halo can be severely misaligned relative to the chrome pattern and cause inspection problems. Recesses 21 are formed to provide typically a 180xc2x0 phase shift in the substrate 12.
Bearing in mind the problems and deficiencies of the prior art, it is an object of the present invention to provide a method and apparatus to make phase shift masks and, in particular, self-aligned alternating phase shift masks having an anti-reflective coating on the opaque patterns of the mask.
A further object of the present invention is to provide phase shift masks made by the method and apparatus of the invention.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The above and other objects and advantages, which will be apparent to one of skill in the art, are achieved in the present invention which is directed to, in a first aspect, a method for making a phase shift mask comprising the steps of:
supplying a transparent substrate mask having an upper surface and a lower surface wherein the upper surface has open regions, opaque pattern regions and an overlying coating on the opaque pattern regions, with the mask being ready for etching the open regions to form the phase shift mask;
applying a photoresist layer on the upper surface of the mask covering the open and opaque coated regions, the photoresist layer having a defined dose-to-clear exposure level;
exposing the lower surface of the mask with a lower surface energy source in an amount less than the dose-to-clear level thereby exposing all the open regions;
exposing the open regions of the upper surface of the mask which are to be etched to form the phase shift mask to an upper surface energy source in an amount less than the dose-to-clear exposure level and the sum of the amounts of the lower surface energy and upper surface energy being at least the dose-to-clear exposure level;
developing the exposed mask removing the resist from the open regions exposed to both the lower surface and upper surface energy sources; and
etching the developed mask to form the desired phase shift mask.
The lower surface of the mask and the upper surface of the mask may be exposed in any order.
In a further aspect of the present invention the upper surface energy source partially overlaps the coated opaque regions of the transparent substrate mask.
In another aspect of the present invention alternating open regions of the transparent substrate are etched to form an alternating phase shift mask.
In a further aspect of the present invention, an apparatus is provided for making a phase shift mask comprising:
a holding device for holding a transparent substrate mask having an upper surface and a lower surface wherein the upper surface has open regions, opaque pattern regions and an overlying coating on the opaque regions, with the mask being ready for etching the open regions to form a phase shift mask;
application means for applying a photoresist layer on the upper surface of the mask covering the open and opaque coated regions, the photoresist layer having a defined dose-to-clear exposure level;
exposing means to expose the lower surface of the mask with an upper surface energy source in an amount less than the dose-to-clear exposure level thereby exposing all the open regions;
exposing means to expose the open regions of the upper surface of the mask which are to be etched to form the phase shift mask with an upper surface energy source in an amount less than the dose-to-clear exposure level and the sum of the amounts of the lower surface energy and the upper surface energy being at least the dose-to-clear exposure level;
developing means to develop the exposed mask to remove the resist from the open regions exposed to both the lower surface and upper surface energy sources; and
etching means to etch the developed mask to form the desired phase shift mask.
In a further aspect of the present invention the upper surface source partially overlaps the corresponding coated opaque regions.
Phase shift masks made by the method and using the apparatus of the present invention are also provided herein.
In another aspect of the present invention a method is provided for making integrated circuit components such as chips comprising the steps of:
supplying an integrated circuit substrate containing a photoresist on the surface thereof;
positioning a mask having the desired circuit design over the substrate, the mask being made by the method of the invention as described above;
exposing the mask to light to expose the substrate in the desired circuit design;
developing the substrate to form the desired circuit pattern; and
forming the desired circuit pattern on the substrate.